Recently, redundancy is widely used in the process control area. The redundancy provides two modules, one of which is an active module and the other of which is a passive module, that is, a backup module. The two modules run the same software, while in the case of normal operation, only one module operates as the active module, and the other module operates as the backup module. When the active module detects a fault, a switch (fail-over) will be started, such that the original active module operates as the backup module, and the original backup module operates as the active module.
In prior art, above switching is achieved by a redundancy line and a communication line. However, by such prior art, it is impossible to determine whether there is any fault in the redundancy line and communication line, which may cause the switching cannot be performed. In process control, such situation may cause losing field control and thus is very dangerous in some scenes.